i'll give that a shot.. what exactly does that do?"If"
i'm not wrong (i'm not a networking expert)
DHCP server "BOOTPS" have as src port 67, then if you block ANY (0.0.0.0/0) traffic coming from your Clients to the WLAN interface of your AP, from port 67, then you are Blocking ANY external DHCP server.
About MikroTik, i can't help you. We are only using it as "Access Concentrator" (fancy name for a PPPoE server) and giving our customers "Static IPs", so i have no experience with MT and DHCP server / DHCP Relay
Also we are planning to take out the MikroTik PPPoE Server from our network (due the fact that Ubiquiti cant do QoS on encrypted traffic) and use Static IPs on the CPEs (in Router mode), and connect our APs (in Bridge mode) directly to the pfSense server.